I think that if you eliminate the fuel injection from the 305 it will negate the mileage gains that you are looking for. The power to weight with 454s in these trucks results in better gas mileage than with a carbureted small blocks. Why don't you just put the money into low pressure fuel...
Run what you have access to manifolds flow well, the ram horn style flow a little better. I picked up a fancy set of 50 state legal shorties from Edelbrock. But I only picked them up because I have a Edelbrock top end and fuel pump and wanted everything to match. In retrospect, long tubes flow...
Here you go. JL 13" shallow truck sub with custom box to spec. with the seat all the way back. JL 600W Amp. Stinger Capacitor felt like I bought a bigger amp and stopped the dash from blinking when I turn it up loud. Kenwood 4x9s and Kenwoods in the dash too. If I could change anything it would...
Squarebody recipe:
If you are gonna stay 12bolt back 10/12 bolt front with a 2-5inch lift I prefer 31-33s. At 3.08 the 33s will bog it down. 3.73s will wake it up and will run well with 35s.
If you do the 3/4 setup go 17s with 375-305s < 4.10.
If you do the one tons - crank out some cash; then...